View Full Version : How is n2 (and all other cryogenic liquids) liquified?
wr0x2
04-12-2005, 05:25 PM
OK, so I know that n2, o2 etc are extracted from liquified air by fractional distillation (correct me if im wrong.) How do they achieve low enough temps to liquify air? Do they use multi stage cascades?
Does anyone have a detailed description of the process, I'm curious.
sjohnson
04-12-2005, 07:24 PM
Here's a schematic: http://www.uigi.com/cryodist.html
wr0x2
04-13-2005, 02:51 AM
Very interesting, thanks for the link. Does anyone have more information on the type of refrigeration used to produce such low temps?
Micutzu
04-13-2005, 04:18 AM
They don't liquify air with the help of low temps, but with the help of high pressure. The air is compressed so much that it becomes liquid, cooling itself in the process. Then it's heated up little by little, and at different temperatures different components evaporate.
wr0x2
04-13-2005, 11:39 AM
I thought that O2, N2, and Ar all had critical temps way below 0c? Doesn't that mean that there would have to be some powerful cooling, especially with the heat generated during compression?
gkiing
04-14-2005, 09:51 AM
They don't liquify air with the help of low temps, but with the help of high pressure. The air is compressed so much that it becomes liquid, cooling itself in the process. Then it's heated up little by little, and at different temperatures different components evaporate.
compression adds heat, it wouldn't cool it.
cadaveca
04-14-2005, 09:58 AM
compression adds heat, it wouldn't cool it.
So you are saying you heat the coolant before is cools? :confused: i thought compression removed heat...you get added heat during the process, as the heat has to go somewhere, but the compressed gas is cooler than it orignally was, was it not? exothermic reaction, maybe?
gkiing
04-14-2005, 10:01 AM
compressing gas heats it up, and compression is not a chemical reaction. I'm not sure how/if they cool it.
cadaveca
04-14-2005, 10:10 AM
I know it's not a chemical reaction, but to increase the density of a gas, energy must be removed, not added. Compression is so, increasing it's density.
In a gas, the molecules move very quickly, but as you increase the density, they become closer together, and when they hit each other, energy is given off in the form of heat, so yes, you COULD say that heat is greater, but this is heat GIVEN OFF, not absorbed. If heat was absorbed, the molecules would move faster, and prevent compression. Maybe it's different at critical temps, but that applies to all the oilfield(my specialty)... and what the basic principle is used for phase-change....the colder gas hits the head, and as it expands, it absorbs heat, taking it away from the processor. How else would it work?
I know it's not a chemical reaction, but to increase the density of a gas, energy must be removed, not added. Compression is so, increasing it's density.
In a gas, the molecules move very quickly, but as you increase the density, they become closer together, and when they hit each other, energy is given off in the form of heat, so yes, you COULD say that heat is greater, but this is heat GIVEN OFF, not absorbed. If heat was absorbed, the molecules would move faster, and prevent compression. Maybe it's different at critical temps, but that applies to all the oilfield(my specialty)... and what the basic principle is used for phase-change....the colder gas hits the head, and as it expands, it absorbs heat, taking it away from the processor. How else would it work?
increasing the pressure makes the gas hot. think of it as friction between the atoms: friction generates heat.
Phase-change gets the evaporator cold by boiling the coolant, thus draining energy from the surroundings since vaporizing something needs alot of energy.
cadaveca
04-14-2005, 11:05 AM
increasing the pressure makes the gas hot. think of it as friction between the atoms: friction generates heat. So you re-interate what i say... :nono: :confused: yes, the gas gets hotter, but you do not ADD heat...it is "exothermic" in nature.
Phase-change gets the evaporator cold by boiling the coolant, thus draining energy from the surroundings since vaporizing something needs alot of energy.
exactly...to opposite of compressing the gas. I've been out of university for a while, but this stuff is pretty basic. We pump raw oil down the pipeline, and regularly cool it to decrease it's size in the pipe(uh, is that not compression?), as if it expands too much, it will create backpressure, which, with such fluids, is highly dangerous. We use helixchangers to cool the oil as it flows.
if ya think you heat things to make them smaller, ya'all need to stay in school.
So you re-interate what i say... :nono: :confused: yes, the gas gets hotter, but you do not ADD heat...it is "exothermic" in nature.
so what you are saying is: if you add heat to a gas with constant volume the pressure is not increased????
you know that's wrong...
...this stuff is pretty basic. We pump raw oil down the pipeline, and regularly cool it to decrease it's size in the pipe(uh, is that not compression?)
you do not increase the pressure by cooling it, you decrease the volume. things get smaller when cold (except water). THAT'S basic...
compressing something means increasing the pressure, not decreasing its volume.
cadaveca
04-14-2005, 11:34 AM
compressing something means increasing the pressure, not decreasing its volume.
you sure about that? compression, by defination, is decreasing volume by phsyical means.
pressure, temperature, and volume of a gas are all relative. This is not regualar thermal properties of a substance, but of gas.
Maybe you better check into this:
http://www.burtoncorblin.com/BCTB101.pdf
it's the basics of gas compression. We ARE talking about gas here. I'm not arguing with for any other reason, than that you are wrong.
BTW, the change between phases is still considered a chemical reaction. Especially in the case of fractional distillation. Did i mention fractional distillation is part of how we refine raw crude?
so what you are saying is: if you add heat to a gas with constant volume the pressure is not increased????
you know that's wrong...
yes, you are correct, as long as the volume stays the same. But the volume is decreased....
you sure about that? compression, by defination, is decreasing volume by phsyical means.
yes, you are of course right...i've been thinking of pressurizing and not compressing. kind of confusing words for a swede ;)
cadaveca
04-14-2005, 11:56 AM
LMAO...never thought about there being a language barrier! :slap: to myself!
:horse:
hehe :) ...my posts makes more sense now, don't they? ;)
cadaveca
04-14-2005, 12:27 PM
hehe :) ...my posts makes more sense now, don't they? ;)
Yes, you are thinking in terms of a cascade, where it is a closed system. Fractional distallation is not.
Further heat transfer, in brazed aluminum heat exchangers, cools the air to cryogenic temperature (approximately -300 degrees Fahrenheit or -185 degrees Celsius). The cooling is accomplished with cold product and waste gas streams exiting the separation process. These exiting gas streams are warmed to close-to-ambient air temperature, which reduces the amount of refrigeration that must be produced in the process. The very cold temperatures needed for cryogenic distillation are created by a refrigeration process that includes expansion of one or more elevated pressure process streams.
-taken from the earlier link, and to answer the question.
wr0x2
04-14-2005, 04:06 PM
OK, I found this (http://lnglicensing.conocophillips.com/NR/rdonlyres/FBB538DA-256D-4B96-A844-5D147F4441CF/0/quartercentury.pdf) pdf. Scroll down to the middle where it has a schematic of the ln2 production process. It cleared up a lot for me.
sjohnson
04-14-2005, 05:39 PM
OK, I found this (http://lnglicensing.conocophillips.com/NR/rdonlyres/FBB538DA-256D-4B96-A844-5D147F4441CF/0/quartercentury.pdf) pdf. Scroll down to the middle where it has a schematic of the ln2 production process. It cleared up a lot for me.Look at the last stage. Anyone here have plans to build a cascade using methane as the refrigerant? :D j/k
wdrzal
04-14-2005, 05:45 PM
Compression by definition is decreasing volume while raising pressure. Read the"perfect gas law" for most of your answers.
cadaveca
04-14-2005, 06:34 PM
That's what i linked earlier..the perfect gas law. I only post the goodness.
ya'all may want to check this site as well:
http://eml.ou.edu/Physics/module/thermal/pasumarthi/firstlaw.html
back to school! You forget the "perfect gas law" does not apply to gases under pressure and must be compensated for!
wdrzal
04-14-2005, 08:43 PM
yea back to school,pressure is always present unless you are in a perfect vacuum. Walt
cadaveca
04-14-2005, 09:03 PM
really? what's a vacuum? There's still pressure in a vacuum.
wdrzal
04-14-2005, 09:13 PM
There is nothing in a perfect vacuum. don't confuse a perfect vacuum with a vacuum reading in gauge preasure. you need to do some reading on the difference of gauge pressure and absolute pressure. walt
cadaveca
04-14-2005, 09:18 PM
LoL. i know what it is. hehe. What is a vacuum? If you want to get deeply technical, you gotta use the right terms! :p: :fact:
:toast:
uh, but is it psia, or psiv? i cannot remember?
Anyway, the process is not a closed system, so there is pressure..you're getting off base a bit here. They use propane, and various other gases to cool the air, or whatever other gas they stip the N2 from. Isn't just the propane enough to extract the N2, and then they use other gases for further compression and storage? They compress it by cooling it further, do they not? Remember, that they do not extract the N2 by compressing it...you can't really compress by pressure in an open system.
wdrzal
04-14-2005, 10:09 PM
That's what i linked earlier..the perfect gas law. I only post the goodness.
ya'all may want to check this site as well:
http://eml.ou.edu/Physics/module/thermal/pasumarthi/firstlaw.html
back to school! You forget the "perfect gas law" does not apply to gases under pressure and must be compensated for!
first you say "perfect gas law does not apply to gases under pressure"
I say if there is gas ther is pressure.
wdrzal
04-14-2005, 10:12 PM
really? what's a vacuum? There's still pressure in a vacuum.
then you say ther still pressure in a vacuum.
I say there is no pressure in a perfect vacuum, you said it in black and white
ps it's not deeply technical :fact: its the laws of physics and thermodynamics
cadaveca
04-15-2005, 06:09 AM
LoL. You miss the point. what does that have to do with the thread starter's question? He asked how they get N2 out of the air...from the start, i have said that they don't use pressure, that they don't compress it by adding heat, that they cool it.
You, walt, of course, are right, but what does it have to do with the topic at hand? So, rather than touting you knowledge about thermodynamics, and how they apply in other circumstances, why don't you stay on topic?
My questions have been puposeful...wrong on purpose, to get the proper answers...it's called leading a discussion. Ever heard of that before?
the perfect gas law only applies in a perfect vacuum, and when they harvest N2, it is not done in a perfect vacumm. So the law must be compensated for. Remember that all my comments are directed at the question, not physics and thermodynamics in a general sense...this is specific.
sjohnson
04-15-2005, 08:37 AM
A perfect vacuum is, by definition, the absence of any matter in a 3 dimensional space.
Add even one atom to that space and the vacuum isn't perfect. It doesn't matter what that atom (or molecule) is.
cadaveca
04-15-2005, 08:43 AM
LoL. I realize that, but there is no absolute vacuum in this process...
wdrzal
04-15-2005, 09:15 AM
A perfect vacuum is, by definition, the absence of any matter in a 3 dimensional space.
Add even one atom to that space and the vacuum isn't perfect. It doesn't matter what that atom (or molecule) is.
This is correct,and a perfect vacuum is not acheivable on earth. so that make the perfert gas laws pertain to to all gase here on earth.